This invention relates to a submerged turbine aeration apparatus.
In the past, a number of different devices have been used for the aeration of liquid waste in activated sludge plants. Three typical methods of aeration are diffused aeration, surface aeration and submerged turbine aeration.
The criteria used in evaluating and comparing aeration equipment is discussed in a paper by John R. Stukenberg and Ross E. McKinney in the January, 1966 issue of the Journal WPCF. In this paper, it is pointed out that the oxygen transfer is the greatest when there is a rapid surface change at the air-liquid interface. According to the two-film theory of oxygen transfer, the oxygen is transferred from gas bubbles to the liquid film and is then transferred by diffusion to the bulk liquid.
The most important design parameter for aeration equipment is the oxygen transfer rate coefficient per hour or K.sub.L a. For standard conditions of one atmosphere pressure and 20.degree. C., the oxygen transfer rate coefficient per hour is referred to as K.sub.L a.sub.20. The value of K.sub.L a is determined by plotting the natural log of the oxygen deficit against time. This results in a straight line with a slope equal to the negative value of K.sub.L a.
In the standard non-steady state method for laboratory testing of aeration equipment, additions of 0.05 mg/l of cobalt chloride are recommended to catalyze the reaction of sodium sulphide with dissolved oxygen in the water.
Employing conventional analysis of data, Stukenberg and McKinney reveal K.sub.L a.sub.20 values of 8.4 for a diffused aerator, 19.3 for a surface aerator, 13.2 for a single submerged turbine and 11.0 for dual submerged turbines.
Aerators and agitators of various types are found in the following U.S. Pat. Nos.
975,380, T. Berntson PA1 3,207,313, K. L. Schulze PA1 3,053,390 F. J. Wood PA1 2,987,186 D. W. Burgoon et al PA1 720,513 T. J. Cheney PA1 1,028,869 A. Fay PA1 3,149,071 D. W. Burgoon et al PA1 1,853,067 O. J. T. G. R. Martin
As seen in the Martin patent, it has been known for some time that air can be introduced into the lower impeller portion of an agitator or the like by providing a hollow shaft, apertures in the shaft above the surface of liquid for the intake of air and apertures in the shaft adjacent the impeller where the air is mixed with the liquid. The use of aerators of this general type for treating waste materials is disclosed, for example, in the two Burgoon patents. In these patents, air is drawn through a hollow shaft and discharged through apertures at the ends of the arms of the impeller. However, Burgoon does not provide efficient mixing of the liquid and air because the liquid in his tank is not recirculated past the air outlets, but is drawn only through a sludge-return pipe. In Schulze, air is drawn towards the impeller through a hollow shaft, but, again, there is no adequate means of recirculating the liquid in the tank past the air outlets. Instead, Schulze relies on the outward velocity imparted to the air bubbles. In Wood, air is introduced into the impeller by means of a draft tube. However, Wood introduces the water into the impeller by means of an elbow and conduit connected to the bottom of the impeller. Since the water is introduced in this way, recirculation of the liquid in the tank past the air outlets is considerably restricted.